Method for the enzymatic conversion of a phenol substrate into a corresponding catechol product

ABSTRACT

A method for the enzymatic conversion of a phenol substrate into a corresponding catechol product comprises the step of incubating the phenol substrate with a  Ralstonia solanacearum  tyrosinase enzyme, or a functional derivative thereof, in a reaction mixture, for a period of time sufficient to allow the enzyme convert at least some of the phenol substrate into the catechol product.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No. PCT/EP2015/079177, filed on Dec. 9, 2015, which claims the benefit of Great Britain Application No. 1422508.0, filed on Dec. 17, 2014. The contents of both applications are hereby incorporated by reference in their entirety.

INTRODUCTION

Tyrosinase is a type of polyphenol oxidase enzyme widely distributed in nature and has several biological functions and biotechnological applications. In the presence of molecular oxygen, tyrosinase catalyses two types of reaction: the hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of o-diphenols to o-quinones (diphenolase activity). The reactive quinones autopolymerize (non-enzymatically) to the macromolecular melanins which are responsible for skin and hair pigmentation, browning of fruit and wound healing in plants and arthropods. Tyrosinase has broad substrate specificity and can accept many types of phenols and diphenols (catechols). Among the substituted phenols it can act on 3- and 4-substituted phenols but 2-substituted phenols are competitive inhibitors of the enzyme.

The ability of tyrosinases to convert monophenols into o-diphenols has motivated studies regarding the production of various o-diphenols which are important precursors for the synthesis of pharmaceuticals, agrochemicals, flavours, polymerization inhibitors, inks, and antioxidants. Halocatechols are interesting synthons due to their biological activity as well as a variety of cross-coupling and halogen-metal exchange reactions starting from halocatechol intermediates to enable the synthesis of functionalized catechol building blocks. For example, fluorocatechol is potentially a valuable precursor for synthesis of pharmaceuticals, such as the adrenergic catecholamines and biogenic amines. Synthesis of substituted catechol by chemical means is complicated due to the employment of aggressive reagents, severe reaction conditions and poor yield. While tyrosinase has great potential as a biological means of synthesizing these catechols, the use of tyrosinases for catechol synthesis has been limited due to the low ratio of monophenolase to diphenolase activity where the accumulation of catechols is not favoured.

Hydroxytyrosol is a highly desired antioxidant used in food, cosmetics, and medicine. The European Food Safety Authority released a statement in 2011 supporting certain claims made about the health benefits of hydroxytyrosol. The price of hydroxytyrosol is high due to a difficult production process (e.g. extraction from olive leaves, chemical synthesis, and extraction from olive oil mill wastewater). The purity of the majority of hydroxytyrosol containing product is low (<80% and often less than 30%) with phenolics and other contaminants present. The low quality, high cost is limiting the application of hydroxytyrosol. The use of a biocatalyst such as tyrosinase would allow for high quality hydroxytyrosol.

A chemical process for production of hydroxytyrosol from tyrosol has been reported but with only 50% conversion at 6 mM concentration (EP1623960). Whole cells of P. aeruginosa can transform tyrosol to hydroxytyrosol (80-96% yield at 25 mM) but produces undesired byproducts (p-hydroxyphenylacetic acid and 3,4 dihydroxyphenylacetic acid) which increases the complexity and cost of the downstream process (Allouche et al., Appl. Environ. Microbiol., 2004, 70, pp 2105-2109; Boullagui and Sayadi, J. Agric. Food Chem., 2006, 54, pp 9906-9911) Furthermore P. aeruginosa is an opportunistic pathogen which adds cost to biological control procedures within a production facility. It is unlikely to be used to produce food additives such as hydroxytyrosol.

ES2320505 describes a process for obtaining L-DOPA from L-tyrosine using the enzyme tyrosinase NP_518485 from the bacterium Ralstonia solanacearum. Molloy et al (Biotechnol. Bioeng. 2013, 110, pp 1849-1857) describes an engineered tyrosine enzyme from Ralstonia solanacearum for improved catalytic efficiency towards D-tyrosine using random and site directed mutagenesis.

US2003180833 (D1) describes the bioconversion of tyrosol into hydroxytyrosol using a mushroom-derived tyrosinase enzyme. While the process provides for high yields, the reaction times were very slow, with 1 g tyrosol conversion using 15 mg of mushroom tyrosinase in a one litre reaction requiring 5 h to complete the reaction. Moreover, preparations of commercial mushroom tyrosinase are inhibited above 10 g/l of tyrosol and cannot complete the reaction. Scientific literature reports inhibition of mushroom tyrosinase by ascorbic acid at 5 mM (Golan-Goldhirsh and Whitaker, J. Agric. Food Chem., 1984, 32, pp 1003-1009; Marin-Zamora et al, J. Biotechnol., 2009, 139, pp 163-168). The Ralstonia solanacearum tyrosinase can act as a biocatalyst as a whole cell, crude cell lysate or purified enzyme. The first two methods of biocatalyst preparation are easy and offer advantages over mushroom tyrosinase used in a purified preparation. It has been suggested that repeated batch system for the conversion of tyrosol (5 g/l) to hydroxytyrosol is possible (Bouallagui and Sayedi J. Agric. Food Chem., 2006, 54 (26), pp 9906-9911) but yields are at 85% in a single run and biocatalyst loses 60% of its activity after 3 runs resulting in poor yields, low overall concentrations and leaving high concentrations of the substrate in the final product. Complicated downstream processing to achieve a highly pure hydroxytyrosol will thus be needed. Furthermore the repeated cycles is cumbersome. Another variation on the fed batch method claims repeated product removal with beads (Brouk and Fishman, J Mol Catal B: Enzym 84:121-127) but this resulted in low product concentration and less than 50% yield.

It is an object of the invention to overcome at least one of the above-referenced problems.

STATEMENTS OF INVENTION

The Applicant has identified a tyrosinase enzyme from Ralstonia solanacearum that is capable of converting tyrosol into a hydroxytyrosol at a very high concentration (up to 150 mM and rate (up to 9.3 g/l/h), compared to any other biocatalyst (best at 25 mM and 0.4 g/l/h) (FIGS. 2-4). The conversion rates of tyrosol to corresponding catechols using Ralstonia solanacearum tyrosinase enzyme is far higher than the conversion rate for L-tyrosine, the natural substrate. The Applicant has also demonstrated that the Ralstonia solanacearum tyrosinase is capable of converting 4-halophenols into corresponding 4-halocatechols with conversion rates 4 g/l/h and product yields (>92%) that are higher than previously reported using tyrosinase or any other enzyme (FIGS. 5A and 5B). The method of the present invention requires 12.9 mg of tyrosinase from R. solanacearum to complete 1 g tyrosol biotransformation in a one litre reaction in 20 min (15 times faster than commercial mushroom tyrosinase). Furthermore the method of the invention demonstrates reactions at high concentration of substrate that heretofore have not been achieved. The Ralstonia solanacearum tyrosinase demonstrates high tolerance to substrate (27.6 g/L of tyrosol) and product (30.8 g/L of hydroxytyrosol). No inhibition of Ralstonia solanacearum tyrosinase activity by ascorbic acid sodium salt (up to 400 mM) was observed. The Ralstonia solanacearum tyrosinase can act as a biocatalyst as a whole cell, crude cell lysate or purified enzyme. The first two methods of biocatalyst preparation are easy and offer advantages over mushroom tyrosianse used in a purified preparation.

Accordingly, the invention provides a method for the enzymatic conversion of a phenol substrate into a corresponding catechol product, the method comprising the steps of incubating the phenol substrate with a Ralstonia solanacearum tyrosinase enzyme, or a functional derivative thereof, in a reaction mixture, for a period of time sufficient to allow the enzyme convert at least some of the phenol substrate into the catechol product.

Preferably, the phenol substrate is a 3-substituted or 4-substituted phenol and the catechol product is a corresponding 3-substituted or 4-substituted catechol.

Preferably, the phenol is a hydroxyalkyl-substituted phenol. Preferably the phenol is a hydroxyethyl-substituted phenol. Preferably, the phenol is a 3- or 4-hydroxyalkyl-substituted phenol. Preferably, the phenol is a 3- or 4-hydroxyethyl-substituted phenol.

Typically, the 4-hydroxyethyl-substituted phenol is tyrosol and the 4-substituted catechol is 3,4-dihydroxyethylphenol.

Suitably, the 3-substituted or 4-substituted phenol substrate is a 3-halophenol or 4-halophenol, and the catechol product is a 3-halocatechol or 4-halocatechol.

Preferably, the reaction mixture comprises ascorbic acid. Preferably, the reaction mixture comprises 100 mM to 300 mM ascorbic acid. In one embodiment, the reaction mixture comprises up to 400 mM ascorbic acid. In one embodiment, the ascorbic acid is provided as a salt, preferably sodium ascorbate.

Suitably, the functional derivative of Ralstonia solanacearum tyrosinase enzyme comprises an engineered enzyme selected from the group consisting of RVC10, RV145 and C10_N322S, the details of which are described in Molloy et al., Biotechnol. Bioeng. 2013, 110, pp 1849-1857. In one embodiment, the functional derivative is the engineered enzyme RV145.

In one embodiment, the Ralstonia solanacearum tyrosinase enzyme, or functional derivative thereof, is provided as a whole cell, crude cell lysate or purified enzyme. In one embodiment, the enzyme is immobilised to a support.

Typically, the substrate concentration is at least 10 mM. In one embodiment, the substrate concentration is at least 100 mM. In one embodiment, the substrate concentration is at least 150 mM. In one embodiment, the substrate concentration is at least 175 mM.

Suitably, the method involves a step of separating catechol product from the enzyme. In one embodiment, the separation step comprises a membrane filtration step. In one embodiment, the separation step comprises a solvent extraction step. In one embodiment the separation is performed using a solid phase to bind and extract the catechol product from the aqueous biotransformation medium.

In one embodiment, the enzyme converts 90% of the phenol substrate into the catechol product. In one embodiment, the enzyme converts 100% of the phenol substrate into the catechol product.

The invention also relates to a catechol product made according to the method of the invention. In one embodiment, the catechol product is selected from hydroxytyrosol and a halocatechol. In one embodiment, the halocatechol is selected from a 3-halocatechol and a 4-halocatechol.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Hydroxylation of halophenols by the monophenolase activity and oxidation of halocatechols by the diphenolase activity of tyrosinase

FIG. 2: Biotransformation of 20 mM of tyrosol in shaking flasks using WT and RV145 enzymes (20 μg/mL) in the presence of 40 mM ascorbic acid sodium salt (TY: tyrosol, HT: Hydroxytyrosol).

FIG. 3A: Biotransformation of 75 nM tyrosol to hydroxytyrosol by purified engineered RV145 enzyme in the presence of 150 mM ascorbic acid sodium salt.

FIG. 3B: Biotransformation of 100 nM of tyrosol using cell free lysate of E. coli harbouring engineered RV145 enzyme in the presence of 200 mM ascorbic acid sodium salt

FIG. 4: 150 mM biotransformation of tyrosol using crude cell free lysate of E. coli harbouring engineered RV145 enzyme in the presence of 300 mM ascorbic acid sodium salt

FIG. 5A: Biotransformation of 10 mM of 4-fluorophenol in shaking flasks using WT and RV145 enzymes (20 μg/mL) in the presence of 20 mM ascorbic acid sodium salt.

FIG. 5B: Biotransformation of 10 mM of 4-iodophenol in shaking flasks using WT and RV145 enzymes (10 ug/mL) in the presence of 20 mM ascorbic acid sodium salt (4-IP: 4-iodocphenol, 4-IC: 4-iodocatechol, 4-FB:4-fluorophenol, 4-FC:4-fluorocatechol)

FIG. 6: 100 mM biotransformation of tyrosol using commercial mushroom tyrosinase in the presence of 200 mM ascorbic acid. The reaction yields only 30% product and stops once 30 mM product is formed showing its much poorer performance compared to R. solanacearum tyrosinase.

DETAILED DESCRIPTION OF THE INVENTION Definitions

“Phenol substrate” means an unsubstituted or substituted phenol, especially a 3-substituted or 4-substituted phenol. Examples of substituents include halogens or hydroxyalkyl substituents. Examples of hydroxyalkyl substituents include hydroxymethyl and hydroxyethyl substituents, especially 4-hydroxymethyl and 4-hydroxyethyl substituents. Typically, the 3-substituted or 4-substituted phenol is a 3-halophenol or 4-halophenol. Examples of halophenols include iodophenol, bromophenol, chlorophenol, and fluorophenol. Preferably, the phenol is tyrosol. Preferably, the phenol substrate is provided at a concentration up to their maximum solubility in aqueous environment. In one embodiment, the phenol substrate is selected from tyrosol, a 3-halophenol, and a 4-halophenol. In one embodiment, the phenol substrate is selected from tyrosol and a 4-halophenol.

“Corresponding catechol product” means a catechol that is produced by reacting a phenol substrate with a Ralstonia solanacearum tyrosinase enzyme. When the phenol substrate is a 4-halophenol (for example, 4-fluorophenol), the corresponding catechol product is the corresponding 4-fluorocatechol (4-fluorocatechol). Likewise, when the phenol substrate is 4-substituted hydroxyalkyl phenol (for example, tyrosol), the corresponding catechol product is the corresponding 4-substituted hydroxyalkyl catechol (hydroxytyrosol).

“Tyrosol” refers to 4-(2-hydroxyethyl)phenol.

“Hydroxytyrosol” means 4-(2-hydroxyethyl)-1,2-benzenediol.

“Ralstonia solanacearum tyrosinase enzyme” means a tyrosinease enzyme isolated from Ralstonia solanacearum. An example of such an enzyme is described in Molloy et al., Biotechnol. Bioeng. 2013, 110, pp 1849-1857. The amino acid and nucleic acid sequences for the wild-type enzyme are provided below:

Ralstonia solanacearum tyrosinase enzyme amino acid sequence (SEQ ID NO: 1) (NP_518458) MVVRRTVLKAIAGTSVATVFAGKLTGLSAVAADAAPLRVRRNLHGMKMDD PDLSAYREFVGIMKGKDQTQALSWLGFANQHGTLNGGYKYCPHGDWYFLP WHRGFVLMYERAVAALTGYKTFAMPYWNWTEDRLLPEAFTAKTYNGKTNP LYVPNRNELTGPYALTDAIVGQKEVMDKIYAETNFEVFGTSRSVDRSVRP PLVQNSLDPKWVPMGGGNQGILERTPHNTVHNNIGAFMPTAASPRDPVFM MHHGNIDRVWATWNALGRKNSTDPLWLGMKFPNNYIDPQGRYYTQGVSDL LSTEALGYRYDVMPRADNKVVNNARAEHLLALFKTGDSVKLADHIRLRSV LKGEHPVATAVEPLNSAVQFEAGTVTGALGADVGTGSTTEVVALIKNIRI PYNVISIRVFVNLPNANLDVPETDPHFVTSLSFLTHAAGHDHHALPSTMV NLTDTLKALNIRDDNFSINLVAVPQPGVAVESSGGVTPESIEVAVI Ralstonia solanacearum tyrosinase enzyme nucleic acid sequence (SEQ ID NO. 2) (gi|30407127) TCAAATGACGGCGACCTCGATCGATTCGGGCGTCACGCCGCCGCTGCTCT CCACGGCAACGCCGGGTTGGGGTACGGCCACCAGGTTGATCGAAAAGTTG TCGTCCCGGATGTTGAGCGCCTTCAGCGTGTCGGTCAGGTTCACCATGGT CGACGGCAGGGCATGGTGGTCGTGTCCCGCCGCATGCGTCAGGAAGCTGA GCGAGGTGACGAAGTGCGGGTCGGTTTCCGGCACATCGAGGTTGGCGTTC GGCAGGTTGACGAAGACCCGGATGCTGATCACGTTGTAGGGGATCCTGAT GTTCTTGATCAGGGCCACGACTTCGGTGGTACTGCCGGTACCAACATCGG CACCCAGGGCACCCGTCACGGTGCCGGCCTCGAACTGGACGGCGCTGTTG AGCGGTTCGACCGCCGTGGCAACCGGATGTTCCCCCTTCAGCACGCTGCG CAGCCGGATATGATCGGCCAGCTTGACGCTGTCGCCGGTCTTGAACAGGG CCAGCAGATGCTCGGCACGGGCGTTGTTCACCACCTTGTTGTCGGCGCGC GGCATGACGTCATAGCGGTAGCCCAGCGCCTCGGTGCTCAGCAGATCGCT CACGCCTTGCGTGTAGTACCGGCCCTGCGGATCGATGTAGTTGTTGGGGA ACTTCATGCCCAGCCACAGCGGGTCAGTCGAGTTCTTGCGGCCCAGCGCG TTCCAGGTGGCCCATACCCGGTCGATATTGCCGTGGTGCATCATGAACAC CGGGTCGCGCGGCGAGGCGGCGGTGGGCATGAAGGCGCCGATGTTGTTGT GGACGGTGTTGTGCGGCGTGCGCTCCAGGATGCCCTGGTTGCCGCCTCCC ATCGGCACCCATTTGGGGTCGAGGCTGTTCTGTACCAGCGGCGGCCGGAC CGAGCGGTCGACCGAACGGCTGGTGCCGAAGACTTCGAAGTTGGTTTCGG CATAGATCTTGTCCATGACCTCCTTCTGGCCGACGATGGCGTCGGTGAGC GCGTAGGGGCCGGTCAGCTCATTCCGGTTGGGCACGTAGAGCGGGTTCGT CTTGCCGTTGTAGGTCTTGGCGGTGAAGGCTTCGGGCAGCAGGCGGTCTT CGGTCCAGTTCCAGTACGGCATGGCGAAGGTCTTGTAGCCGGTGAGCGCG GCCACGGCGCGCTCGTACATCAGCACGAAGCCGCGGTGCCAGGGCAGGAA GTACCAGTCGCCGTGCGGGCAGTACTTGTAGCCGCCGTTGAGCGTACCGT GCTGGTTGGCAAAGCCGAGCCAGCTCAGCGCCTGCGTCTGGTCCTTGCCT TTCATGATGCCGACGAACTCGCGATAGGCCGACAGGTCCGGGTCGTCCAT CTTCATGCCATGCAGGTTGCGCCGCACGCGCAGCGGGGCGGCATCGGCCG CAACAGCGGAGAGGCCGGTCAGCTTGCCCGCGAATACCGTGGCGACACTT GTCCCGGCGATTGCCTTCAGCACCGTTCTACGCACGACCAT

“Functional derivative thereof” as applied to a Ralstonia solanacearum tyrosinase enzyme means an engineered variant of Ralstonia solanacearum tyrosinase enzyme that is typically capable of converting a phenol (i.e. tyrosol) into a corresponding catechol (i.e. hydroxytyrosol) at a concentration and rate that is significantly better than mushroom tyrosinease described in US2003180833 (D1). Examples of such engineered variants of Ralstonia solanacearum tyrosinase enzyme are described in Molloy et al (2013), including variants that are capable of converting tyrosol into hydroxytyrosol at a concentration and rate that is at least the equivalent of the wild-type Ralstonia solanacearum tyrosinase enzyme. In one embodiment, the engineered variant of Ralstonia solanacearum tyrosinase enzyme is capable of the 100% conversion of 150 mM tyrosol into hydroxytyrosol in the tyrosol biotransformation assay described below. In one embodiment, the engineered variant of Ralstonia solanacearum tyrosinase enzyme is capable of the 100% conversion of 175 mM tyrosol into hydroxytyrosol in the tyrosol biotransformation described below. Methods for generating and testing engineered variants of Ralstonia solanacearum tyrosinase enzyme will be apparent to the person skilled in the art, and are described in Molloy et al.

An “engineered variant” of the Ralstonia solanacearum tyrosinase enzyme protein shall be taken to mean enzymes having amino acid sequences which are substantially identical to wild-type Ralstonia solanacearum tyrosinase enzyme. Thus, for example, the term should be taken to include enzymes that are altered in respect of one or more amino acid residues. Preferably such alterations involve the insertion, addition, deletion and/or substitution of 5 or fewer amino acids, more preferably of 4 or fewer, even more preferably of 3 or fewer, most preferably of 1 or 2 amino acids only. Insertion, addition and substitution with natural and modified amino acids is envisaged. The engineered variant may have conservative amino acid changes, wherein the amino acid being introduced is similar structurally, chemically, or functionally to that being substituted. Generally, the variant will have at least 70% amino acid sequence homology, preferably at least 80% sequence homology, more preferably at least 90% sequence homology, and ideally at least 95%, 96%, 97%, 98% or 99% sequence homology with wild-type Ralstonia solanacearum tyrosinase enzyme. In this context, sequence homology comprises both sequence identity and similarity, i.e. a polypeptide sequence that shares 70% amino acid homology with wild-type Ralstonia solanacearum tyrosinase enzyme is one in which any 70% of aligned residues are either identical to, or conservative substitutions of, the corresponding residues in wild-type Ralstonia solanacearum tyrosinase enzyme. Specific variants included within the scope of the invention are the engineered variants described in (Molloy et al., 2013), especially (RVC10, RV145 and C10_N322S). In one embodiment, the engineered variant of Ralstonia solanacearum tyrosinase enzyme is capable of complete conversion of a tyrosol substrate to hydroxytyrosol at concentrations an order of magnitude higher than previously reported by any tyrosinase or other biocatalyst (e.g. 175 mM tyrosol)

The term “engineered variant” is also intended to include chemical derivatives of wild-type Ralstonia solanacearum tyrosinase enzyme, i.e. where one or more residues of the wild-type enzyme is chemically derivatized by reaction of a functional side group. Also included within the term variant are wild-type Ralstonia solanacearum tyrosinase enzymes in which naturally occurring amino acid residues are replaced with amino acid analogues. Examples of side chain modifications include modification of amino groups, such as by reductive alkylation by reaction with an aldehyde followed by reduction with NaBH₄; amidation with methylacetimidate; acetylation with acetic anhydride; carbamylation of amino groups with cyanate; trinitrobenzylation of amino groups with 2, 4, 6, trinitrobenzene sulfonic acid (TNBS); alkylation of amino groups with succinic anhydride and tetrahydrophthalic anhydride; and pyridoxylation of lysine with pyridoxa-5′-phosphate followed by reduction with NABH₄. The guanidino group of arginine residues may be modified by the formation of heterocyclic condensation products with reagents such as 2,3-butanedione, phenylglyoxal and glyoxal. The carboxyl group may be modified by carbodiimide activation via o-acylisourea formation followed by subsequent derivatization, for example, to a corresponding amide. Sulfhydryl groups may be modified by methods, such as carboxymethylation with iodoacetic acid or iodoacetamide; performic acid oxidation to cysteic acid; formation of mixed disulphides with other thiol compounds; reaction with maleimide; maleic anhydride or other substituted maleimide; formation of mercurial derivatives using 4-chloromercuribenzoate, 4-chloromercuriphenylsulfonic acid, phenylmercury chloride, 2-chloromercuric-4-nitrophenol and other mercurials; carbamylation with cyanate at alkaline pH. Tryptophan residues may be modified by, for example, oxidation with N-bromosuccinimide or alkylation of the indole ring with 2-hydroxy-5-nitrobenzyl bromide or sulphonyl halides. Tryosine residues may be altered by nitration with tetranitromethane to form a 3-nitrotyrosine derivative. Modification of the imidazole ring of a histidine residue may be accomplished by alkylation with iodoacetic acid derivatives or N-carbethoxylation with diethylpyrocarbonate. Examples of incorporating unnatural amino acids and derivatives during enzyme synthesis include, but are not limited to, use of norleucine, 4-amino butyric acid, 4-amino-3-hydroxy-5-phenylpentanoic acid, 6-aminohexanoic acid, t-butylglycine, norvaline, phenylglycine, ornithine, sarcosine, 4-amino-3-hydroxy-6-methylheptanoic acid, 2-thienyl alanine and/or D-isomers of amino acids.

EXPERIMENTAL

Materials and Methods

Strains, Growth Conditions and Purification of Tyrosinase

Recombinant E. coli BL21 were used to express wild type (WT) tyrosinase gene from R. solanacearum and three engineered variants (RV145, RVC10, C10_N322S) of the same gene previously generated by Molloy et al (2013). All strains were maintained in Lysogeny broth (LB) with 50% glycerol at −80° C. In preparation for induction and tyrosinase purification, the E. coli strains stored at −80° C. were streaked on LB agar and incubated at 37° C. for 24 h. The primary inoculum was cultured in a rotary shaker (200 rpm, 37° C.) in 5 mL of LB overnight. Carbenicillin (50 μg/mL) was used as the antibiotic for the maintenance of the plasmid throughout the study. Tyrosinase production was induced and the enzyme was purified as described earlier (Molloy et al. 2013). Different fractions of the purified tyrosinase were analysed by 10% SDS-PAGE for confirmation of purity and molecular weight. The protein concentration was determined by bicinchoninic acid method (Smith et al. 1985) using bovine serum albumin as the standard.

Preparation of Substrates and other Chemicals

Various monophenol substrates such as 4-fluorophenol, 4-bromophenol, 4-chlorophenol, and 4-iodophenol were purchased from Sigma-Aldrich (Dublin, Ireland). Stock solutions (1 M) of the 4-bromophenol, and 4-iodophenol were prepared in 100% ethanol and subsequent working solutions were further diluted in 50 mM potassium phosphate buffer (pH 7). Similarly, stock solutions (500 mM) of 4-fluorophenol and 4-chlorophenol were prepared in deionized water and working stocks were prepared in 50 mM phosphate buffer. Tyrosol was purchased from TCI Europe (Belgium) and stocks were prepared in 50 mM phosphate buffer. Ascrobic acid and ascorbic acid sodium salt (AA) (used to reduce o-quinone to o-diphenol) was prepared in deionized water freshly before the biotransformation assay

Biotransformation of 4-halogenated Phenols and Tyrosol

Biotransformations of halophenols into corresponding halocatechols were carried out using purified enzymes (WT and engineered variants). Various concentrations (5, 10, and 20 mM) of halophenol biotransformations were carried out in 100 mL conical flasks with a working volume of 20 mL at 30° C. and 200 rpm in a shaker incubator. Biotransformation of tyrosol to hydroxytyrosol was carried out using purified enzymes, cell free lysate or whole cells. Various concentrations (75, 100, 150 and 175 mM) of tyrosol biotransformations were carried out in 500 mL baffled conical flasks with a working volume of 100 mL at 30° C. and 200-250 rpm in a shaker incubator. An enzyme concentration of 2 μg/mL per 1 mM of substrate was tested with a substrate to sodium ascorbate ratio of 1:2. Biotransformation of 100 mM tyrosol to hydroxytyrosol was also carried out using commercial mushroom tyrosinase (Sigma, Ireland) using the above specified conditions. Samples (450 μL) were withdrawn at various time points and immediately added to 50 μL of ice-cold 1N HCl. Samples were centrifuged (12,000 g), filtered using Whatman Mini-UniPrep™ syringeless filters (0.45μ Whatman Inc. NJ, USA), and a sample volume of 20 μL was used for all HPLC injections.

HPLC Analysis

Filtered samples were analyzed by HPLC using a 5 μm ACE 5 C18column (25 cm×4.6 mm ID; Apex Scientific, Ireland) and a Hewlett-Packard (Palo Alto, Calif., USA) HP1100 instrument equipped with an Agilent 1100 Series Diode Array Detector. The samples were isocratically eluted at 22° C. using a phosphoric acid (0.1%, v/v) and methanol mix at a flow rate of 1.0 mL/min. The ratio of phosphoric acid to methanol was 70:30 for 4-fluorophenol and 50:50 for 4-chloro-, 4-bromo-, and 4-iodophenol. 

The invention claimed is:
 1. A method for the enzymatic conversion of tyrosol into hydroxytyrosol, the method comprising the steps of incubating at least 75 mM tyrosol with a Ralstonia solanacearum tyrosinase enzyme, or a functional derivative thereof, in a reaction mixture, for a period of time sufficient to allow the enzyme to convert at least 90% of the tyrosol into hydroxytyrosol and in which the reaction mixture comprises at least 150 mM ascorbic acid, wherein the functional derivative is an engineered variant of Ralstonia solanacearum tyrosinase selected from the group consisting of Y119F, V153A, D317Y and L330V (RV145); T1831, F185Y, N322S, and T359M (RVC10); and N322S (C10_N322S).
 2. A method according to claim 1 in which the ascorbic acid is provided as a salt.
 3. A method according to claim 2 in which the ascorbic acid is provided as a sodium salt.
 4. A method according to claim 2 in which the functional derivative of the Ralstonia solanacearum tyrosinase enzyme is capable of the 100% conversion of 175 mM tyrosol into hydroxytyrosol.
 5. A method according to claim 1 in which the functional derivative of the Ralstonia solanacearum tyrosinase enzyme is capable of the 100% conversion of 175 mM tyrosol into hydroxytyrosol.
 6. A method according to claim 1 in which the functional derivative of Ralstonia solanacearum tyrosinase enzyme is RV145.
 7. A method according to claim 1 in which the Ralstonia solanacearum tyrosinase enzyme, or functional derivative thereof, is provided as an extract from a bacteria that expresses the Ralstonia solanacearum tyrosinase enzyme, or functional derivative thereof.
 8. A method according to claim 1 in which the Ralstonia solanacearum tyrosinase enzyme, or functional derivative thereof, is provided as a bacteria that expresses the Ralstonia solanacearum tyrosinase enzyme, or functional derivative thereof.
 9. A method according to claim 1 in which the Ralstonia solanacearum tyrosinase enzyme, or functional derivative thereof, is provided as a purified enzyme.
 10. A method according to claim 1 in which the Ralstonia solanacearum tyrosinase enzyme, or functional derivative thereof, is provided as a crude mixture in the form of a cell extract.
 11. A method as claimed in claim 1 in which the hydroxytyrosol is separated from impurities present in the reaction mixture by chilling the reaction mixture after the enzymatic conversion step.
 12. A method as claimed in claim 1 in which the hydroxytyrosol is separated from impurities present in the reaction mixture by refrigerating or freezing the reaction mixture after the enzymatic conversion step.
 13. A method according to claim 1 in which the functional derivative of Ralstonia solanacearum tyrosinase enzyme is RVC10.
 14. A method according to claim 1 in which the functional derivative of Ralstonia solanacearum tyrosinase enzyme is C10_N322S. 